Page 87 - Master Handbook of Acoustics

An Experiment in Pitch

The level of sound affects the perception of pitch. For low frequencies, the pitch goes down as the
level of sound is increased. At high frequencies, the reverse takes place; the pitch increases with
sound level.
The following is an experiment suggested by Fletcher. Two audio oscillators are required, as well
as a frequency counter. One oscillator is applied to the input of one channel of a playback system, the

other oscillator to the other channel. The frequency of one oscillator is adjusted to 168 Hz and the
other to 318 Hz. At low level these two tones are quite discordant. If the level of the 168-Hz and 318-
Hz tones is increased until the (perceived) pitches decrease to 150 Hz and 300 Hz, this will yield an
octave relationship which gives a pleasant sound. This illustrates the decrease of pitch at lower
frequencies. A similar test would show that the pitch of higher frequency tones increases with sound
level.

The Missing Fundamental

The auditory system can sometimes play tricks on our perception of sound. If tones such as 1,000;
1,200; and 1,400 Hz are reproduced together, a pitch of 200 Hz is heard. This can be interpreted as
the fundamental with 1,000 Hz as the fifth harmonic; 1,200 Hz as the sixth harmonic; and so on. The
auditory system recognizes that the upper tones are harmonics of the 200-Hz tone and perceptually
supplies the missing fundamental that would have generated them.

Timbre versus Spectrum

Timbre describes our perception of the tonal quality of complex sounds. The term is applied chiefly
to the sound of musical instruments. A flute and oboe sound different even though they are both

playing the same pitch. The tone of each instrument has its own timbre. Timbre is determined by the
number and relative strengths of the instrument’s partials.
Timbre is a subjective term. The analogous physical term is spectrum. A musical instrument
produces a fundamental and a set of partials (or harmonics) that can be analyzed with a wave
analyzer. Suppose, for example, the fundamental frequency is 200 Hz, the second harmonic frequency
is 400 Hz, the third harmonic is 600 Hz, and so on. The subjective pitch that the ear associates with

our measured 200 Hz varies slightly with the level of the sound. The ear also has its own subjective
interpretation of the harmonics. Thus, in an intricate way, the ear’s perception of the overall timbre of
an instrument’s note might be considerably different from the measured spectrum. In other words,
timbre (a subjective description) and spectrum (an objective measurement) are not the same.

Localization of Sound Sources

The perception of the location of a sound source begins at the external ear, with the pinna. Sound
reflected from the ridges, convolutions, and surfaces of the pinna combines with the unreflected direct
sound at the entrance to the auditory canal. This combination, now encoded with directional
information by the pinna, passes down the auditory canal to the eardrum and then to the middle and